It's the greek alphabet - Some variants were labeled but did not become widespread, so those letters were skipped, as was Xi (probably due to Winnie the Pooh).
alpha
beta
gamma
delta
epsilon
zeta
eta
theta
iota
kappa
lambda
mu
nu
xi
omicron
pi
rho
sigma
tau
upsilon
phi
chi
psi
omega
It's the greek alphabet - Some variants were labeled but did not become widespread, so those letters were skipped, as was Xi (probably due to Winnie the Pooh). alpha beta gamma delta epsilon zeta eta theta iota kappa lambda mu nu xi omicron pi rho sigma tau upsilon phi chi psi omega
Thanks!